Most people infected with Borrelia burgdorferi (Bb) will develop Lyme disease, which accounts for >90% of all vector-borne illnesses in the United States, and e $1-2 billion each year in direct medical expenses and lost productivity. This infection elicits potent innate and adaptive immune responses that often fail to clear the bacteria, leading to persistent infection and disease in targeted tissues. Early recognition of Bb lipoproteins through Toll-like receptor 2 on innate immune cells appears to be crucial for bacterial clearance from host tissues, however we currently have no clear understanding of the particular innate responses that are critical for preventing Lyme disease. Our previous studies revealed that IL-10-deficient mice (IL-10-/-) control Bb levels in host tissues significantly better than wild type (WT) mice, thus making IL-10 the only cytokine shown to significantly affect Bb clearance. Our preliminary data further indicate that 1) viable (but not killed) Bb elicit high IL-10 levels from murine macrophages (MXs) and dendritic cells (DCs), 2) that the secreted levels of IL-10 can significantly inhibit subsequent activation of resident MXs, 3) that significant IL-10 levels are detected in mouse skin within 24h of infection, and 4) that IL-10-/- mice do not exhibit these suppressive effects, leading to enhanced MX activity and immune clearance. We hypothesize that the IL-10 elaborated in response to Bb infection, suppresses host innate immune responses that are crucial for efficient Bb clearance from host tissues, and that MX and DC functions are central to this dysregulated host response. The overall goals of this proposal are: 1) to identify the cell types that initially detect and respond to Bb in murine tissues, 2) to determine which cell types are responsible for producing IL-10 during the course of infection, and 3) to delineate which immune cell types are adversely affected by Bb-elicited IL-10 and which immune mechanisms are dysregulated. To approach these goals, Aim 1 will compare IL-10 effects on the abilities of MXs and DCs to ingest, internally traffic, and mediate killing of Bb in vitro, including the expression of activation/maturation markers. Aim 2 will utilize a number of genetically mutated mouse lines to delineate the relative importance of hemopoietic and nonhemopoietic cells, as well as MXs, DCs, and T cells to both produce IL-10 and/or respond to IL-10 during Bb infection in vivo. Aim 3 will rigorously test whether administration of IL-10 receptor-blocking antibodies could be used as a curative therapy for mice infected with Bb. These studies should clarify specific host immune mechanisms that are suppressed during the development of Lyme disease, leading to the development of novel treatments. PUBLIC HEALTH RELEVANCE Infection with B. burgdorferi causes host tissues to significantly increase IL-10 production, which suppresses the host immune responses and hinders their ability to clear this infection, leading to the development of Lyme disease. IL-10 is the only cytokine that is currently known to significantly affect the development of Lyme disease and represents an important clue into the immune evasion mechanism of this pathogen. These studies intend to identify the host cells that produce IL-10, as well as the immune cells that are dysregulated by IL-10, thus identifying mechanisms that could be targeted to treat Lyme disease patients.